The Callitrichaceae are an aquatic family of dicots that include the single, geographically cosmopolitan genus Callitriche. Calliltriche contains 40-50 terrestial, amphibious, and oligately submersed species, and it is the only known genus in the plant kingdon with co-occurring serial and hydrophilous pollination syndrome. Pollen morphology and ultrastructure were described for 13 Callitriche species using scanning electron and transmission electron microscopy. Representative taxa of each growth form were examined; these included three terrestrial species (C. deflexa, C. peploides, and C. mutalli), nine amphibious species (C. brutia, C. cophocarpa, C. cophocarpa-stignalis hybrid, C. cribosa, C. haumulata, C. heterophylla var heterophylla, C. lusitanica, C. marginata, and C. trochlearis), and one obligately submerged species (C. truncata). Of the amphibious taxa, C. heterophylla var. heterophylla and C. trochlearis had internal geitonogamy, a type of internal self-fertilization. Pollen from all taxa was spheroidal, small, intectate, and lacked well-defined apertures. Taxa primarily differed with respect to exine thickness, surface ornamentation, and the presence or absence of aperture-like regions. The pollen of terrestial species, as well as that of C. marginata, had well-developed exines with thick sculpted and basal layers. In general, amphibious taxa produced pollen with distinct, but thinner, exines than that of terrestrial taxa. Pollen of the amphibious taxa with internal geitonogamy had a thicker basal layer than species with internal geitonogamy, whereas the overall exine was reduced in C. hamulata and absent in C. brutia and C. lusitanica. Pollen of the obligately submersed C. truncata also lacked an exine. These palynological data were correlated with growth habits and related pollination biologies, as well as with phylogenetic interpretations of Callitrachacaea. Exine reduction or loss has evolved at least twice in the family, and it is associated with aneuploid reduction in chromosome number.